A diver photographs a Humboldt squid (Dosidicus gigas) in its native waters off the coast of Mexico. The jumbo squid, which can grow six feet long, has been appearing as far north as Alaska. Scientists theorize that changes in oxygen levels in the Pacific Ocean have allowed the squid to flourish in new waters.

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The toothed suckers of the Humboldt squid, which can reach 110 lbs. Its presence and voracious appetite in northern waters may threaten fish stocks, such as hake.

When large numbers of jumbo squid first showed up in California’s Monterey Bay in 1997, scientists weren’t sure what had brought the cephalopod that far north. An unusually strong El Niño event had warmed the eastern Pacific. But the squid, dubbed el diablo rojo – the red devil – in its native waters off the coast of Mexico, didn’t typically venture farther north than Baja California.

And indeed, within two years, the Humboldt squid – Dosidicus gigas – had disappeared from central California waters.

But in 2002 – another El Niño year – they reappeared. This time, they took up permanent residence and pushed even farther north – past Oregon, Washington, and British Columbia, until, by 2004, fishermen near Sitka, Alaska, were hauling them in.

When scientists dug through historical records, they discovered that the squid’s northward advance wasn’t entirely unprecedented. There were accounts from the 1930s of the creatures in Monterey Bay. But never in numbers comparable to what scientists observed now – schools many hundreds strong. And no one had ever seen them as far north as Alaska.

Fishermen worry that the squid, a voracious predator weighing up to 110 pounds and reaching more than six feet in length, will diminish valuable fish stocks.

Hake, for example, a major Pacific fishery, has declined since the squid arrived.

Scientists, meanwhile, ponder what the dramatic range expansion of a species usually confined to lower latitudes implies about the Pacific Ocean in general.

They’re gradually piecing together a story of natural cycles that, together with climate change, have altered the eastern Pacific in a way that favors jumbo squid.

Oxygen-depleted waters expand

Originally, some thought that the squid were growing more numerous because overfishing had reduced their predators and competitors, such as tuna. But in the absence of concrete evidence of overfishing, scientists have since looked to changing environmental conditions for an explanation.

Now, they think the squid are moving both north and south from their equatorial stronghold – they’ve also become more abundant in Chilean waters – because conditions they’re uniquely adapted to – low-oxygen, or hypoxic, water at a certain depth – are expanding poleward from the tropics as well.

An oxygen-depleted layer of water exists naturally many hundreds of feet below the ocean surface. But for the past 50 years in the Pacific Ocean, this layer has become less saturated with oxygen and moved upward. At depths between 656 and 1,640 feet, areas of the north Pacific have lost between 1 and 2 percent of their oxygen each year during the past 25 years, says Frank Whitney, a scientist emeritus with Fisheries and Oceans Canada in Sidney, British Columbia. And the top edge of this low-oxygen zone has advanced upward at an average rate of almost 10 feet per year.

Most sea life that has gills prefers to avoid these hypoxic waters. For these species, the ocean has effectively become 246 feet shallower in the past quarter century. This may explain why some fish species off the coast of British Columbia have moved to shallower areas, and, in some cases into Alaskan waters, says Mr. Whitney. “I would suggest that would be in response to hypoxia.”

How ocean waters lose oxygen

Some of the deep water along the west coast of North America originates off the coast of equatorial South America, where the water is already “old,” meaning that it hasn’t been in contact with the atmosphere for many years. And it’s further leached of oxygen when organic detritus drops from the highly productive surface waters of the tropical eastern Pacific. As this organic material sinks, it decays, sucking oxygen from the water and creating one of the largest hypoxic zones in the world.

There are two possible explanations for this. An ocean that warms from the top down becomes stratified, like a layer cake. The warmer and more buoyant surface then inhibits oxygenation of cooler waters below. The second possibility: increased productivity. More organic material means more decay and more oxygen removed from the ocean. Some of that oxygen-depleted water then flows north.

Paradoxically, however, productivity in the eastern Pacific is highest when the cycles influencing surface temperatures are in their cool phase. That’s when nutrient-rich water wells up from the deep, fertilizing the algae that ultimately support one of the most productive marine ecosystems on Earth. Recently the ocean has entered a “cool” phase of the 20- to 50-year cycle called the Pacific Decadal Oscillation – warmer waters in the western tropical Pacific, but cooler ones in the eastern tropical Pacific. “When we’re in the cold phase of the PDO, we have lower oxygen,” says Dr. Chavez.

What thriving squid may indicate

Others have also noted a link among increased productivity, hypoxia, and squid abundance. Longtime fishermen in Mexico’s Sea of Cortez report that there were far fewer squid there in the past, says William Gilly, a professor of biology at Stanford University in Palo Alto, Calif. He suspects that agricultural development inland and the large quantities of fertilizer now dumped by rivers into the mostly enclosed sea have caused algal blooms, worsened hypoxia, and made the sea more inviting to the opportunist squid.

“The fertilizers and the oxygen minimum zone go in parallel, and then at some point the [numbers of] squid just explode,” he says.

The question remains, however, about what’s causing the hypoxic zone to expand not just locally but across the entire eastern Pacific. Whitney doubts that it’s higher productivity caused by fertilization.

“We see this hypoxia because the ocean and atmosphere are not exchanging oxygen as well as they used to,” he says.

He suspects that changing conditions in the sub-Arctic Pacific where deep water is created are at fault. In winter, oxygenated surface waters near Japan and Russia become cold and dense enough to sink. But intensified stratification, he thinks, has disrupted this process.

Rising temperatures have probably contributed, but Whitney also suspects a freshwater influx, a phenomenon observed in parts of the North Atlantic.

Low-density fresh water has probably capped the ocean, inhibiting the exchange of gases between ocean and atmosphere and impeding deep-water formation. Melting arctic ice and permafrost have probably increased freshwater flow into the north Pacific.

Rain could also be having an impact. Climate models predict that, in a warmer world, rainfall will increase at certain latitudes. More water evaporates from warmer tropical seas, and a warmer atmosphere holds more moisture. But eventually this water must fall from the sky somewhere. Some, Whitney suspects, falls at the higher latitudes of the sub-Arctic Pacific. “You keep doing that decade after decade, there’s going to be some impact,” he says.

Although natural cycles are probably behind some of the changes in the Pacific Ocean that scientists are observing, climate change seems to be pushing the ocean beyond the limits of natural variability. The jumbo squid invasion of California and beyond is one symptom of these larger oceanographic changes.